A projector suitable for the projection of digital images typically includes a light source, aspect ratio optics and projection optics. The light source may be LEDs (light emitting diodes), lasers or other high-intensity light source. The aspect ratio optics controls the aspect ratio of the projected image, e.g. a 4:3 aspect ratio (i.e. conventional TV) or a 16:9 aspect ratio (i.e. “wide screen” TV). The aspect ratio optics could include two polygonal mirrors, oriented at right angles to each other. Each mirror is rotated at a controlled speed along an axis perpendicular to all of that mirror's facets. A beam of light from the light source hits a first facet of the first rotating mirror when the facet is at a precisely selected angle, and is then reflected into a second facet on the second rotating mirror. Accordingly, the first rotating mirror may be used to cause the beam to sweep rows of pixels from left to right across a screen, while the second rotating mirror may be used to deflect the beam to result in a plurality of rows from the top of the screen to the bottom of the screen. Thus, the two rotating mirrors result in the aspect ratio of the image. Light leaving the aspect ratio optics passes through projection optics which focuses the light on the screen.
One issue governing the use of rotating polygonal mirrors in the aspect ratio optics is the number of facets to include on each mirror. The smaller the number of facets, the greater the “sweep” of the beam, i.e. the greater the range by which the angle of the beam may be adjusted, and the wider or taller the projected image can be, and the more pixels it can include, at a given distance from the screen and using a given type of projection optics. However, the larger the number of facets, the more frequently the image may be refreshed. Accordingly, during the design phase of the projector, the number of facets on each of the two polygonal mirrors may be selected to result in a preferred ratio of the width of the horizontal and vertical sweeps, balanced by a preferred refresh rate.
The above issue results in a problem, in that the ratio of the height and width of data representing an incoming image may not be the selected ratio of the width of the horizontal and vertical sweeps. That is, the ratio of the height to width of the image data to be projected may not be optimized by the sweeps resulting from the selected number of facets on the mirrors within the aspect ratio optics. This problem may be solved by turning off the light beam for a portion of the time. For example, where the polygonal mirrors were selected to result in a 4:3 aspect ratio, and where data associated with the incoming image is a 16:9 aspect ratio, the top and bottom of the projected image may be eliminated. That is, since the width is fixed, the height must be reduced to conform to the ratio of height to width of the incoming data. The unused top and bottom portions of the screen are familiar to people having 4:3 televisions who watch wide screen movies. Thus, while the polygonal mirrors are scanning through the top and bottom of the screen, the light beam is turned off. Similarly, where the selected ratio of the width of the horizontal and vertical sweeps of the polygonal mirrors was selected for a wider, shorter image, (e.g. widescreen, 16:9) and the incoming data represented a narrower, taller (e.g. 4:3) then vertical strips on the left and right of the screen could be turned off. This is the case where a widescreen 16:9 TV is used to watch conventional 4:3 images.
The above solution, i.e. turning off the light beam to produce unused vertical or horizontal strips, results in an image having the aspect ratio of the incoming data. However, several problems remain. For example, the overall size of the image, due to the unused strips, is smaller than the image the projector is capable of producing. A related issue is that there are a number of pixels which, but for the fact that they were turned off, could have been transmitting information to a viewer. Since these pixels are unused, the viewer receives no benefit from them. Thus, the projector is conveying less data to the user than it would if the aspect ratio of the data were different. Additionally, in part because some of the pixels have been turned off to allow the image to fit within the available viewing area, the incoming data may have to be reduced in resolution to allow projection within the available area; i.e. the incoming data may have to be “dumbed down” in resolution to be displayed with the pixels available, while other pixels remain unused.
A second solution to the problem requires less (or no) reduction in the resolution of the incoming data. This solution is to project as much of the image as will fit, while cropping portions of the image which requires a sweep that is greater than that sweep which one of the polygonal mirrors may provide. (Recall that a polygonal mirror with more facets has less angle of sweep than a polygonal mirror with less facets, but has a greater refresh rate and is capable of making each pixel, to which it can sweep, brighter.) For example, where the projector is suited, due to the sweep associated with the number of facets selected, for projection of a 4:3 image, the central portion of a 16:9 image may be projected, and the left and right sides cropped. This allows the portion of the image projected to be at a maximum level of resolution. However, due to the cropping of the image, some data is not displayed to the viewer.
Accordingly, the viewer would benefit from improved projection systems.